Hardening of Metal Alloys

ABSTRACT

A method of rendering an alloy with increased hardness is provided, the method including the steps of heat treating the alloy until the formation of at least one ordered region in the alloy. An alloy prepared according to this method and an item of jewelry fashioned from or including such an alloy are also provided.

BACKGROUND OF THE INVENTION

This invention relates to alloy metals. In particular, this invention relates to a method of rendering an alloy metal with increased hardness. The invention has particular relevance to platinum alloys and hardening of low-solute platinum alloys by heat treatment.

Pure platinum is too soft (Vickers Hardness Value (HV) of around 45) to survive wear and tear as a jewelry item. For this reason all platinum jewelry is made from platinum alloyed with other elements, because the addition of the other elements results in an alloy of increased HV. There is a limit to alloying additions, however, the limits being imposed by hallmarking requirements.

Internationally, jewelry can only be hallmarked as “platinum” if it contains a minimum platinum value, for example, 85%, 90% or 95% platinum by weight (wt. %). Different countries have different requirements. In South Africa, for example, platinum jewelry must contain 95% by weight platinum in order to secure the platinum hallmark. The addition of up to 5 wt. % of other elements to platinum can improve its HV (e.g. platinum 5 wt. % tungsten has a HV of 135) but the hardness nevertheless remains low relative to other metals (e.g. mild steel has a HV of 140 and stainless steel a HV of 250+). In this specification ‘low-solute’ alloys are intended to encompass those alloys having 15 wt. % or less solute.

The hardness of an alloy can be further improved by cold work defined as plastic deformation below the recrystallisation temperature T_(rx). T_(rx) is material dependent but is usually approximately 1000 degrees Centigrade (deg C) for common platinum jewelry alloys, although this figure can vary by several hundred degrees depending on the material. It will be appreciated that cold working is inherent in hand-working jewelry manufacturing processes. As such, cold working includes rolling, drawing, etc of the alloy material. For example, cold worked platinum 5 wt. % tungsten can have a HV of 310.

Another method of hardening alloys known in the art is age-hardening or precipitation hardening, which increases hardness by solid-state precipitation of a second phase. This process involves:

1) an initial high-temperature heat treatment (the temperature depends on the particular alloy), 2) a quench (to room temperature or below), and 3) a second heat treatment, at lower temperature than (1), (again, this temperature depending on the particular alloy).

This process can only be carried out for specific alloys but it can result in HV of up to 350 in commercial, precipitation hardening platinum alloys (known as “heat treatable” platinum alloys).

In 2002, Nzula et al. (Nzula, M. P. and Lang, C. I. Ordering in platinum-chromium alloys, Proc.

ICEM: International Congress on Electron Microscopy, Durban, South Africa (2002)) reported that a cold worked Pt 10 atomic weight (at.) % Cr alloy had an increased hardness (approximately HV of 370) which conventional hardening mechanisms could not account for when heat treated at 300 deg C. It was further reported that an electron diffraction-pattern of the alloy was consistent with the diffraction patterns observed in the Pt₈Ti type structure. However, the equilibrium phase diagram for the PtCr system did not show an ordered phase at this composition.

Although the crystallography and thermodynamics of an X₈Y (where X is a parent metal and Y is a solute atom) ordered structure have been the subject of considerable interest, to date little attention has been paid to the effect of the formation of the X₈Y ordered structure on the properties of these alloys and the effect of the formation of Pt₈Y on the mechanical properties of platinum alloys has not previously been thoroughly investigated.

It therefore remains a goal to produce alloys having increased hardness, over and above that taught by heat treatment.

The reason for these efforts to increase hardness of an alloy, whilst still retaining its hallmark, is that high hardness confers high scratch-resistance and wear resistance, which are desirable characteristics in jewelry items such as rings. Jewelry items such as rings bearing a hallmark will obviously be of higher value commercially.

A need exists to provide a method for rendering an alloy with increased hardness.

SUMMARY OF THE INVENTION

According to a first aspect to the present invention there is provided a method of rendering an alloy with increased hardness, the method including the steps of heat treating the alloy until the formation of at least one ordered region in the alloy, wherein the alloy is not a PtCr alloy.

The term increased hardness refers to the comparison of the alloy including ordered regions and that alloy not including ordered regions. Thus the term ‘increased hardness’ is intended to encompass rendering an alloy with a hardness not predictable or obtainable using conventional hardening mechanisms.

The term ‘ordered region’ is intended to mean the existence of a super-lattice structure, in which the location of different atomic species is periodic and predictable.

Preferably the ordered region is an X₈Y ordered region within the alloy wherein X is a parent metal and Y is the solute atom type.

The parent metal is preferably platinum (Pt). However, other metals such as palladium (Pd) and nickel (Ni) are also considered to fall within the ambit of the present invention. Pd alloys, like Pt alloys, find particular application in jewelry applications and Ni has application in high temperature and other applications.

Preferably the solute atom is a transition metal element. As such the solute atom may be selected from Ti, V, Zr, Cr (where the parent metal is not Pt), Nb, Mo, Hf, Ta and W. The metal alloy may be a binary alloy, i.e. comprising the parent metal and one other solute atom, or it may be a ternary alloy, i.e. comprising the parent metal and two solute atoms. It will be appreciated that quaternary and other alloys are also contemplated to lie within the ambit of the present invention.

The alloy preferably includes at least 85 wt. % parent metal, more preferably at least 90 wt. %, more preferably at least 95 wt. % parent metal. The solute atoms may be present in not more than 15 wt. % more preferably not more than 10 wt. %, more preferably not more than 5 wt. %

In a preferred embodiment of the present invention, the alloy comprises 95 wt. % of the parent metal, 0.01 to 4.99 wt. % of a first solute atom, the balance of the alloy comprising at least one further solute atom selected to enhance predetermined criteria of the alloy.

In a preferred embodiment of the present invention, the parent metal is Pt and the solute atom is V. The alloy preferably contains more than 94 wt. % Pt, more preferably more than 95 wt. % Pt, preferably more than 96 wt. % Pt, most preferably more than 96.5 wt. % Pt. The alloy may contain less than 99.99 wt. % Pt, preferably less than 98 wt. % Pt, preferably less than 97 wt. % Pt, more preferably less than 96 wt. % Pt. The alloy preferably contains less than 6 wt. % V, more preferably less than 5 wt. % V, more preferably less than 4 wt. % V, most preferably less than 3.5 wt. % V. The alloy may contain more than 0.01 wt. % V, more preferably more than 1 wt. % V, more preferably more than 2 wt. % V, more preferably more than 3 wt. % V. In a most preferred embodiment of the present invention, the alloy comprises 95 wt. % Pt, 3.2 wt. % V, the balance being selected from the group of transition metals set out above.

Preferably the alloy is heated to below its recrystallisation temperature (T_(rx)). The alloy is preferably cooled to ambient temperature following heating. The alloy is preferably heated to below the order/disorder temperature (T_(c)) of the alloy to induce ordering. It will be appreciated that a given alloy may present one of two scenarios according to the present invention:

(1) T_(c) is below the recrystallisation temperature T_(rx)—in which case heating below T_(c) means heating below T_(rx). (2) T_(c) is above T_(rx)—in which case it may be required to heat above T_(rx) to induce ordering. However, heating above T_(rx) may negate the effect of any prior cold work with the result that there may be a reduced net hardening effect. In accordance with the present invention, if T_(c) is above T_(rx) the alloy is heated below T_(rx) (which is below T_(c)).

The method may include the step of initially cold working the alloy to a predetermined degree of deformation/strain. In this manner it is possible to achieve a particular hardness prior to heating the alloy as hereinbefore described. This may be achieved by cold working the alloy for a pre-determined period of time at a pre-determined strain rate. The subsequent heat treatment is preferably effected in a furnace.

According to a second aspect to the present invention there is provided an alloy prepared according to the method of the first aspect to the invention.

According to a third aspect to the present invention there is provided an alloy comprising a parent metal selected from Pt, Pd and Ni and a solute atom wherein the alloy has a Vickers Hardness Value of more than 370, preferably more than 400, more preferably more than 420, more preferably more than 440, more preferably more than 450, more preferably more than 470, more preferably more than 490, most preferably more than 500.

According to a fourth aspect to the present invention there is provided an item fashioned or including an alloy as hereinbefore described and/or prepared as hereinbefore described.

According to a fifth aspect to the present invention there is provided the use of an alloy including an ordered region and/or as hereinbefore described in the preparation of an item. The item may be an item of jewelry, cutlery or other valuable.

DESCRIPTION OF EMBODIMENTS

The hard alloys according to the present invention are achieved by the formation of ordered regions within the metal alloy, ‘ordered regions’ meaning the existence of a super-lattice structure, in which the location of different atomic species is periodic and predictable, rather than simply being a random solid solution of different atomic types.

The preferred type of ordered region has the stoichiometry X₈Y, where X is the parent metal and Y is the solute atom type. A preferred alloy is a platinum alloy and Pt₈X is believed to exist in the following alloys set out below. Other preferred alloys include palladium and nickel alloys and the same stoichiometry also exists in these alloys. This has been predicted from first thermodynamic principles known in the art [A. J. Ardell, Metallic alloys: Experimental and theoretical perspectives, 93102 (1994). Z. W. Lu and B. M. Klein, Phys. Rev. B 50, 5962-5970 (1994)].

-   PtTi: the Pt₈Ti structure has been predicted, observed, and reported     in the open literature. -   P. Pietrowski, Nature, 206, 291 (1965). -   PtV: the Pt₈V structure has been predicted, observed and reported in     the open literature D. Schryvers, J. Van Landuyt and S. Amelinckx,     Mat. Res. Bull, 18, 1369 (1983). -   PtZr: the Pt₈Zr structure has been predicted, observed and reported     in the open literature. -   P. Krautwasser, S. Bhan and K. Schubert, Z. Metallkd. 59, 724 (1968) -   PtCr: The PtBCr structure has been predicted and was observed by the     applicant. -   PtNb: the P % Nb structure has been predicted, but not yet observed. -   PtMo: the Pt₈Mo structure has been predicted, but not yet observed. -   PtHf, PtTa, PtW: the Pt₈Hf, Pt₈Ta and Pt₈W structures have been     predicted, but not yet observed.

(It will be appreciated that these latter systems are less urgent because the Pt₈X occurs at above 10 wt. % solute and such an alloy therefore does not meet South African hallmarking requirements.)

Without wishing to be bound by theory, it is believed that the reason that the X_(B)Y structures have been observed so rarely in the art is because the kinetics of the ordering transformation are very slow. The applicants have found that the kinetics can be accelerated by appropriate cold work. It must be appreciated that the acceleration of the kinetics is not essential according to the present invention. If the kinetics for a particular alloy are slow, the kinetics can be accelerated by quenching from high temperatures and/or the kinetics can be accelerated by irradiating the metal alloy.

The applicants have found that the formation of the Pt₈X structure confers an impressive increase in hardness to an alloy—something that has not previously been observed. For example, when the PtV structure forms in the 3.2% .wt PtV alloy, a HV of over 500 is achieved. This is a significant value compared to the HV of cold-worked stainless steel at 400. The above HV of over 500 is particularly surprising considering the HV of the annealed 3.2% .wt PtV alloy is approximately 210 and the HV of the cold worked alloy is approximately 360. (Annealing is heat treatment designed to negate the effect of cold working.)

To achieve this elevated hardness (higher than any other platinum jewelry alloy), the alloys according to the present invention must be heat treated to produce the Pt₈X structure. There are several important practical considerations here:

the method comprises only a single heat treatment, of as short a duration as 15 minutes. No further change in hardness is observed following heat treatments of up to 2500 hours. By comparison, precipitation hardening, which produces the next highest hardening in jewelry alloys, requires two heat treatments, possibly lengthy.

the heat treatment temperatures (which differ from alloy to alloy) are between 300-1000 deg C.,—the slow kinetics of the Pt₈X transformation are greatly accelerated by cold work prior to heat treatment. Since most jewelry processes involve cold work anyway, this would not have to be an additional step.

In other words, once a jewelry item has been made by (cold) hand-working of an alloy, it can simply be placed into a furnace for approximately 15 minutes at a temperature below the T_(rx) for that alloy to induce the formation of X₈Y stoichiometry. This will confer a greater HV to the alloy than heretoknown before and may even confer the alloy with a HV higher than stainless steel.

Further features of a method of hardening low-solute platinum alloys, in accordance with the invention, are described hereinafter by way of the following non-limiting examples and figures.

FIG. 1 is a graph of hardness vs. heat treatment temperature for platinum 3.2 wt. % vanadium alloy heat treated for three hours.

FIG. 2 is a graph of hardness vs. time for platinum 3.2 wt. % vanadium alloy heat treated at 400 deg C.

FIG. 3 is a graph of hardness vs. heat treatment temperature for platinum 2.9 wt. % chromium alloy heat treated for three hours.

FIG. 4 is a graph of hardness vs. time for platinum 2.9 wt. % chromium heat treated at 300 deg C.

FIG. 5 is a graph of hardness vs. heat treatment temperature for platinum 5.2 wt. % molybdenum heat treated for three hours.

In FIGS. 1, 3 and 5, the hardness of the cold worked alloy is shown at heat temperature treatment 0.

Example 1

With reference to FIGS. 1 and 2, a method of manufacturing an item of jewelry from a platinum alloy containing 3.2 wt. % vanadium in accordance with the invention is described hereinafter.

The platinum alloy is cast and homogenised to provide an alloy having a uniform structure. The term ‘uniform structure’ refers to compositional homogeneity and an equiaxed grain structure. The platinum alloy is then cold worked to produce the jewelry item by a process of rolling, drawing or any other suitable means of working, resulting in a HV of approximately HV_(100g) 366. The jewelry item is then heat treated in a furnace at a temperature below 600° C. for three hours, followed by cooling to room temperature.

After the heat treatment the jewelry item has an enhanced HV of approximately 500 which is higher than the HV that can be attained by cold working, i.e. approximately 360.

Example 2

With reference to FIGS. 3 and 4 the platinum/chromium alloy is cast and homogenised to provide an alloy having a uniform structure. The platinum alloy is then cold worked to produce the jewelry item by a process of rolling, drawing or any other suitable means of working, resulting in a HV of approximately HV_(100g) 295. The jewelry item is then heat treated in a furnace at a temperature below 600° C. for three hours, followed by cooling to room temperature.

After the heat treatment the jewelry item has an enhanced HV of approximately 330 which is higher than the HV that can be attained by cold working, i.e. approximately 295.

Example 3

With reference to FIG. 5 the platinum/molybdenum alloy is cast and homogenised to provide an alloy having a uniform structure. The platinum alloy is then cold worked to produce the jewelry item by a process of rolling, drawing or any other suitable means of working, resulting in a HV of approximately HV_(100g) 360. The jewelry item is then heat treated in a furnace at a temperature below 600° C. for three hours, followed by cooling to room temperature.

After the heat treatment the jewelry item has an enhanced HV of over 400 which is higher than the HV that can be attained by cold working, i.e. approximately 360. 

1. A method of rendering an alloy with increased hardness, the method including the steps of heat treating the alloy until the formation of at least one X_(B)Y ordered region within the alloy wherein X is a parent metal and Y is the solute atom type and wherein the alloy is not a platinum/chromium (PtCr) alloy.
 2. A method according to claim 1 wherein the parent metal is selected from platinum (Pt), palladium (Pd) and nickel (Ni).
 3. A method according to claim 1 wherein the solute atom is a transition metal element.
 4. A method according to claim 3 wherein the solute atom is selected from Ti, V, Zr, Cr (where the parent metal is not Pt), Nb, Mo, Hf, Ta and W.
 5. A method according to claim 1 wherein the alloy is a binary alloy.
 6. A method according to claim 1 wherein the alloy is a ternary or quaternary alloy.
 7. A method according to claim 1 wherein the alloy includes at least 85 wt. % parent metal.
 8. A metal according to claim 1 wherein the solute atoms are present in not more than 15 wt. %.
 9. A method according to claim 1 wherein the alloy comprises 95 wt. % of the parent metal, 0.01 to 4.99 wt. % of a first solute atom, the balance of the alloy comprising at least one further solute atom selected to enhance predetermined criteria of the alloy.
 10. A method as claimed in claim 9 wherein the alloy comprises 95 wt. % Pt, 3.2 wt. % V, the balance being selected from the group of transition metals.
 11. A method according to claim 1 wherein the alloy is heated to below its recrystallisation temperature (T_(rx)).
 12. A method as claimed in claim 11 wherein the alloy is cooled to ambient temperature following heating.
 13. A method as claimed in claim 12 wherein the alloy is heated to below an order/disorder temperature (T_(c)) of the alloy to induce ordering.
 14. A method according to claim 1 including the step of initially cold working the alloy to a predetermined degree of deformation/strain.
 15. An alloy prepared according to the method of claim
 1. 16. An alloy comprising a parent metal selected from Pt, Pd and Ni and a solute atom wherein the alloy has a Vickers Hardness Value of more than
 370. 17. An item fashioned from or including an alloy according to claim
 16. 18. Use of an alloy including an X₈Y ordered region wherein X is a parent metal and Y is the solute atom type in the preparation of an item.
 19. Use as claimed in claim 18 wherein X is selected from Pt, Pd and Ni.
 20. An item as claimed in claim 17 wherein the item is an item of jewelry, cutlery or other valuable. 